The present invention relates to a heat roller type fixing apparatus which comprising a heating element such as an infrared lamp having a temperature distribution property so arranged that the highest temperature is obtained at portions of the heat roller between the central portion and both axial portions than at other portions when the heating element is energized, that is, in the rising time of the element and, a temperature sensing element positioned so coming into contact with or closing to the surface of the heat roller at the highest portion thereof.
Various types of apparatus have been used for heat fixing in electro-photographic copying machines, e.g. direct-heating type apparatus, oven type apparatus, roller type apparatus and so forth.
Among these known methods, the roller type fixing apparatus is superior in that it does not cause fire even in case of jamming with copying paper. A typical example of this type of apparatus is disclosed in the specification of U.S. Pat. No. 3,196,766.
This type of fixing apparatus, however, involves a problem concerning the rising time of fixing. Namely, this type of apparatus requires a longer period of time until the roller surface is heated up to a predetermined temperature for heat fixing a toner image onto the copying paper after the start of the electric power supply to the heating element, as compared with other types of apparatus such as, for example, direct heating type. Various studies have been made to shorten the rising time of fixing, in order to obviate the above-mentioned problem in the roller type fixing apparatus. However, even the improved roller type fixing apparatus requires a rising time which is as long as 2 to 5 minutes.
These problems are described by using a conventional roller type fixing apparatus as shown in FIGS. 1 to 3, in detail. FIG. 1 is a perspective view of a heat roller type fixing apparatus, FIG. 2 is a sectional view taken along the line A--A of FIG. 1 and FIG. 3 shows a temperature curve of the heat roller surface thereof. In Figures, a reference numeral 1 denotes a heat or upper roller which is usually an aluminum roller having a diameter of 40 mm and a wall thickness of 4 to 5 mm and coated with a tetrafluoroethylene of 40 to 80.mu. thick. An infrared ray lamp or heating element 2 disposed at the inside of the roller and adapted for heating the roller, in which the lamp has a capacity, for example, of 800 W and the filament is elongated along in the direction of length of the lamp so as to be obtained uniformly temperature distribution when the lamp is energized. A lower roller 3 is adapted to cooperate with the upper roller 2 in clampling and driving the copying paper therebetween. The lower roller 3 usually is made of silicon rubber or coated with the same.
A reference numeral 4 denotes bearings for rotatably supporting the rollers, while gears for rotatively driving the upper and lower rollers are designated at reference numerals 5 and 6. A reference numeral 7 denotes a temperature sensing element constituted by a thermocouple such as of chromel-alumel or a temperature-sensitive resistance element such as thermister, which element is disposed at a central portion of the upper roller. A cover 8 is adapted to protect the temperature sensing element against wind.
In order to shorten the rising time of the fixing device, various measures can be adopted such as to increase the capacity of the infrared ray lamp 2 or to reduce the heat capacity of the upper roller 1. However, if the rising period or time is reduced to 1 minute or so by adopting the aforementioned measures, the overshoot of the temperature, which will be described latter, is inconveniently increased, and the temperature of the upper roller is lowered considerably at both axial ends of the roller as compared with the central portion of the same, resulting in an uneven temperature distribution of the roller and difficulty in the temperature control.
The term "overshoot" of temperature in this specification is used to mean a phenomenon in which, referring to FIG. 3, the roller surface temperature which starts to rise concurrently with the start of the power supply to the lamp 2 in the roller 1, is increased to the level of point B beyond the aimed or set temperature A.
When the temperature sensing element or heat sensitive element 7 is held in contact with the central portion of the upper roller surface, the temperature sensing element delivers a control signal when the roller surface temperature reaches the level A to actuate the control circuit thereby to cut the power supply to the lamp 2. The overshoot or the increase of the temperature to the level B is unavoidable even by this cutting of the power supply to the lamp 2.
This overshoot of the temperature rise is attributable to the temperature gradient existing across the thickness of the upper roller 1, i.e. between the inner surface of the upper roller 1 facing the lamp 2 and the outer surface of the same. Namely, a considerably large amount of heat has been accumulated in the upper roller 1 by the time when the temperature sensing element acts to cut the power supply to the lamp 2 upon detect of the heat roller surface, so that the roll surface temperature is increased even after the cutting of electric power supply. This phenomenon becomes more remarkable as the heat capacity of the lamp 2 is increased.
A larger overshoot will be caused if the temperature sensing element is kept out of contact with the roller, even if the temperature sensing element is set to produce a control signal at a temperature lower than an aimed temperature which is, for example, 180.degree. C. to set the roller surface temperature at the aimed temperature, i.e. 180.degree. C.
Next, the reason why the surface temperature of both ends of the roller 1 is specifically low will be described hereinunder with reference to FIG. 2 showing the roller in section. Firstly, it is to be pointed out that the light-emitting portion of the infrared ray lamp is not extended to the hearing portions 12 of both axial ends. Secondly, the bearing portions 12 themselves have considerably large thermal capacity and are contacted by the bearing 4 and the gear 5 which in turn are thermally coupled to other gears and panels. The phenomenon that the temperature of the upper roller 1 is specifically low at both axial ends as compared with the temperature of the central portion thereof is remarkable particularly in the fixing device which is set to shorten the rising time, partly because of the poor heat conductivity in the axial direction of the upper roller 1 attributable to the thinning of the wall of the latter, and partly because of the fixing device is put into use before the heat is sufficiently distributed to the ends of the roller 1 with the bearing 4 and gear 5 still in cold state, due to the shortened rising time.
Generally speaking, on the one hand, a low fixing temperature causes a fixing under (or insufficient fixing), which is a phenomenon to cause an easy separation of the toner image from the copying paper. On the other hand, a too high fixing temperature causes an offset phenomenon, i.e. a phenomenon in which a part of the toner image is separated from the copying paper and welded to the roller and then transferred to another portion of the copying paper.
The overshoot of the temperature and the unbalance of the temperature distribution on the heat roller surface, if it is not improved, will be caused to inconvenient problems in practice, even if the fixing apparatus which is set to shorten the rising time.